The present invention is directed at a precision ophthalmic surgical laser method and system and includes an active eyetracker system and method for accurate and efficient eyetracking which takes into consideration eye tilt, and laser delivery systems and methods well suited for accommodating eye tilt during laser application.
Roughly two decades ago, surgical techniques were introduced in an effort to permanently correct shortsightedness and astigmatism. The radial keratotomy procedure used a diamond blade to make incisions into the cornea, the front surface or xe2x80x9cwindow of the eyexe2x80x9d. Although this technique worked relatively well, there have been problems with long term stability of vision and weakening of the cornea as a result of the cuts often having to be made up to 95% of the corneal thickness.
More recently, these older techniques have been replaced with laser treatment techniques which have replaced the surgeon""s blade with a computer controlled laser that gently re-sculptures the shape of the cornea without cutting or, for most applications, weakening the eye. These laser techniques are typically carried out with a photoablation process using an excimer laser.
An excimer laser""s extreme accuracy and low thermal effect makes it well suited as an eye laser. Many eye lasers are extremely accurate and remove only 0.25 microns (1/4000th millimeter) of tissue per pulse. During corneal re-sculpturing, the excimer laser gently xe2x80x9cevaporatesxe2x80x9d or vaporizes tissue; there is no burning or cutting involved. In the normal eye, light rays entering the eye are accurately focused on the retina and a clear image is formed. Most of the bending or focusing of the light rays occurs at the cornea, with the natural lens inside the eye being responsible for fine adjustments. If light is not focused on the retina, then the eye is said to have a refractive error. Common refractive errors include: myopia or shortsightedness, hyperopia or farsightedness, and astigmatism. The excimer laser has been used to re-sculpture the cornea in myopia, hyperopia and astigmatism corrections in an effort to make the curvature of the cornea focus light rays normally on the retina.
Presbyopia is a problem considered to be due to an aging process occurring in the natural lens of the eye, and thus does not fall under the same category as the refractive errors of myopia, astigmatism and hyperopia noted above, although combinations of presbyopia and one or more of the refractive errors are possible. U.S. Pat. Nos. 5,533,997 and 5,928,129 to Dr. Luis A. Ruiz describe presbyopia corrective apparatus and methods which involve the use of a laser system to remove tissue from the eye in presbyopic corrective patterns discovered to be effective by Dr. Ruiz. These two patents are incorporated herein by reference. Reference is also made to PCT Publication No. WO 00/27324 for International Application No. PCT/US99/26242 filed on Nov. 8, 1999, directed at improvement in presbyopia xe2x80x9cLASIKxe2x80x9d surgery. This PCT publication is also incorporated herein by reference and represents further improvement in addressing presbyopia by way of laser surgery.
Also, the corneal surface is not a very smooth body and has topographical irregularities which can be both large and small. Under prior art laser systems these surface irregularities are not fully taken into consideration in the standard formulas and patterns designed to correct defects such as hyperopia, myopia and astigmatism. Accordingly, the final ablation profile formed in the eye deviates to some extent from what was predetermined by the surgeon to be the final resultant profile of the eye, and this is particularly true with respect to eyes with highly irregular surfaces wherein the defect can be simply shifted to a lower corneal altitude and thus create a new defect which is often unpredictable under the prior art systems. Reference is made to U.S. Pat. No. 6,129,722 which issued on Oct. 10, 2000 and is incorporated herein by reference. U.S. Pat. No. 6,129,722 describes improvements in eye ablation volume formation in laser eye surgery that takes into consideration the topographic irregularities in the eye being ablated, while also allowing for the input of the surgeons expertise.
Reference is also made to co-pending U.S. Ser. Nos. 09/598,226 and 09/598,227 each filed on Jun. 21, 2000 to Dr. Luis Ruiz and Eduardo Matallana which are incorporated by reference herein. These applications describe means for enhancing accuracy, registration and desired beam density application to conform the applied ablation volume pattern with the desired vision enhancements through use of an active mask in the path of the laser beam.
Despite the above described improvements in determining the desired ablation volume to be applied and providing a laser system capable of achieving high precision with respect to the desired ablation volume pattern, if the laser can not keep up with movement of the eye, including eye tilt about its normal axis, than all the enhancements in these other areas will be lost or degraded in the final result.
Efforts have been made in the prior art to improve the tracking response of a laser with eye movements. Laser systems without an eyetracker system rely on having patients fixate their gaze upon a fixation light. This technique does not, however, prevent rapid movements of the eye. Further, a momentary lapse in fixation can result in an ablation shot far from the intended shot location. As an alternative, physical fixation devices have been used which immobilize the eye by physically connecting to the eye, thereby holding it steady. This technique can lead to increased patent discomfort and a further cluttering of the surgical area.
More recent techniques involve computer aided eyetracking devices. These tracking devices are typically optical or topographic location systems that use a video camera to either optically or topographically locate and track the center of the eye. Examples of such systems can be found in U.S. Pat. No. 5,602,436 to Lang et al., U.S. Pat. No. 5,098,426 to Sklar et al., U.S. Pat. No. 5,162,641 to Fountain and U.S. Pat. No. 4,848,340 to Bille. These systems use various techniques to track the center of the eye, such as a computer mapped digital image from a video camera. For example, U.S. Pat. No. 5,098,426, to Sklar, et al., hereby incorporated by reference, describes an eyetracking system that generates a three dimensional profile of the eye and tracks movement by noting changes in that profile. The Sklar patent shows an eyetracker using a slow control loop and a fast control loop. The slow control loop relies on a video camera to provide topographical information that the eyetracker then uses to aim the system optics.
An alternate eyetracking system is shown in U.S. Pat. No. 4,848,340 to Bille, also incorporated by reference. The Bille patent shows a strictly optical, rather than topographical, based system that tracks a reference grid which has been ablated into the eye. U.S. Pat. No. 5,980,513 to Frey et al. illustrates another example of an eyetracking system and relies on substantial mirror movement requirements.
Some prior art eyetracking systems use infrared light to illuminate the pupil of the eye in an effort to facilitate tracking. One example is found in U.S. Pat. No. 5,620,436 which utilizes an eyetracker system relying on a non coaxial infrared heat source located on a side of a patient for the detection of an infrared heat target normally being the center of the pupil.
The above described systems tend to be either invasive, not particularly accurate and/or complicated, in the sense that they require actual physical markings to be made on the eye ( as shown in the Bille patient), or require highly complex and often not highly accurate topographical location systems and multiple feedback loops for locating the center of the eye. Also, those systems relying on infrared heat sources are very sensitive to changes in the illumination and infrared spectrum contaminations generated by the on/off conditions of the illumination lamps of the microscope and auxiliary lights used by the surgeon during the surgery as well as surgeon generated shadows by his hands and instruments in the operating field or even by the patient morphological constitution.
These illumination changes generated by the on/off and intermediate conditions of the surgical lights when the surgeon uses the microscope usually confuse the eyetracker, oversaturate the video camera, confuse the software and the computer detection algorithms used by these systems creating potentially dangerous situations for the patient and the end result of the surgery.
Another drawback in prior art eyetracker systems are that they are limited by not being able to provide sufficient information in order to have the laser delivery system apply the laser beam precisely where desired with respect to the eye. The deficiencies in the prior art eyetracker systems are especially pronounced when dealing with eye tilting which is a more common situation than X, Y plane shift. The lack of eyetracker systems adequately addressing the problem of tracking eye tilt movement is not surprising in view of the lack of laser delivery systems well suited for accommodating eye tilt. The inability to track eye tilt and provide the appropriate adjustment in the laser beam delivery can lead to undesirable results during the resculpturing of an eye. For example, under a common myopia pattern application wherein a cap is removed based on a basic geometrical ablation volume pattern, there is a tendency in the prior art to induce error in eyes that are tilted at the time of laser application. Much like the seasonal shifts on Earth due to tilting with respect to the sun, the application of a laser beam to a tilted eye induces an unintended variation in energy application over the ablated region such that one portion of the intended ablation pattern receives a greater degree of energy than planned, while another region receives less applied energy than planned. This unintended variation in applied energy due to eye tilting is more pronounced when dealing with large beam laser application systems. An additional problem concerning eye tilt is the potential for an eye tilt to be present at the time of determining initial reference coordinates for video tracking. This initial error in reference determination carries over to all later laser beam applications during an eye ablation process and thus can lead to significant errors. These errors created due to eye tilt clearly appear in a post-op topographical exam.
The present invention features an active eyetracker system, method and apparatus designed and built to sense and detect variations in the (X), (Y) coordinates as well as to detect any eye tilt. With the eye tilt being detected, for example, by a monitoring of any inclination of the normal axis of the eye from an initial state corresponding with the normal of the optical train to a new position forming an angle with respect to the normal of the optical train axis due to pivoting of the eye within its orbital socket. In a preferred embodiment, the optical eyetracker system uses the strategically positioned regular and auxiliary microscope illumination lights to provide reference marking means that take advantage of the natural ability of the cornea to bend and refract all light, central or peripheral in toward the pupil. It is the bent and refracted light beam that establish the reference markings in accordance with the present invention.
Under the present invention eye tilt tracking is possible through use of a tracking system which utilizes reference marking means such as non-invasive light beam pointer marking devices and/or ring illuminator device(s) that project on to the iris of the eye a reference marking pattern that is arranged to provide eye tilt information and is used in conjunction with other eyetracker components such as, for example, beam splitters, a patient fixation light, a surgical microscope with alignment marks on the oculars, a turning mirror, a video camera, a photodiode array sensor, an eyetracker camera (if the video camera is not used for the same), an electronic video frame grabber, and head holding means such as a cervical pillow. With information provided by the eyetracker system, a laser system computer and related detection software enables the present invention to capture process and establish the initial corresponding reference coordinates for the eye and any subsequent shifts of the eye along an X-Y plane and/or eye tilts about the eye""s normal axis.
Active eyetracker operation is facilitated under the present invention in the utilization of two main contrast landmarks of the eye in the context of both reference marking and in analyzing the position of the eye both with respect to X-Y plane shifts and normal axis eye tilts. The two landmarks include, for example, the contour of the iris and the pupil. The iris provides an advantageous location for reflecting the reference marking pattern of the marking means while the pupil center point provides a convenient reference point for comparison purposes with respect to those reference markings for determining whether the eye is tilted. This information is used, for example, in conjunction with reference to the reticula marks on the oculars of the surgeon microscope and the typically corresponding eyetracker camera""s centration marks wherein movement of the eye can be broken down both from the standpoint of eye tilt angle and eye shift along the X-Y plane. Preferably the non-invasive reference marking device utilizes the cornea refraction and iris reflectivity in association with a plurality of auxiliary satellite markers (e.g., illumination lights, ring illumination lights and/or laser pointer lights) which can be used for the initial centration and alignment of the eyetracker and the targeted eye and for tracking of movement of the eye after the initial reference is taken.
Eyetracker detection and correction is based on, comparisons between, for example, initial and subsequent X and Y reference coordinate values for changes associated with the referencing technique of the present information which provides information both as to any eye shift and any eye tilt. For example, by comparing with the laser system computer the digital position information provided with respect to the initial captured image against a later image capture relative to any changes in the reference position parameters providing the eye tilt and shift information, accurate tracking of eye tilt and shift is made possible under the present invention. The eye tilt and shift analysis information is also used under the present invention to compensate the laser beam delivery system to conform to such changes in eye position.
In a preferred embodiment, the eye tilt movements are detected as centration variations of, for example, a central point in the pupil, within surrounding concentrically arranged reference marking points and/or one or more concentric marker rings generated by the non-invasive reference marker means. The reference marker positioning can be mapped and converted to digital position information in accordance with the software associated with the eyetracker camera being utilized for displacement by the computer processor. Data relating to the detected reference pattern provided by the reference marking means through the cornea (and preferably projected on to the iris) and any concentricity variations relative to the center of the pupil center, for instance, with respect to the reference pattern are processed by the computer and the detection software to detect any angle tilt in the normal axis of the eye. The same reference marking means can also be used to detect non-tilt shifts in the eye.
The present invention is also directed at providing a laser delivery system that utilizes the active eyetracker""s shift and tilt information in providing laser beam delivery commands that corrects for any detected shift and/or tilt. All the information provided by the eyetracker system is properly processed via the computer and the detection software to generate compensations for these eye movements via the excimer laser delivery system before the delivery of the next individual excimer laser pulse to the patient cornea to precisely place and deliver every laser pulse in the exact location required by the programmed surgery.
The present invention can also be used in conjunction with conventional laser systems to take advantage of the ability of the present invention to determine eye tilt and differentiate the same from a generally non-tilt shift or a combination. For example, this information is of high importance in the initial video frame reference capture and can also be used to determine if the eye has moved into an unacceptable tilt warranting a no-shot signal to the laser system (e.g. an extreme tilt might warrant a no-shot signal despite the fact that the pupils shift on the X-Y plane falls within acceptable shift parameters for a shot by the laser) The present invention is, however, more preferably used in conjunction with a laser system capable of properly following along with eye tilting to provide essentially equal energy application across the desired laser beam contact area(s).
The present invention further features a laser delivery system that includes an optical assembly which is capable of accommodating movements in the eye and particularly eye tilt variations. In one embodiment of the invention, the optical assembly includes means for delivering an excimer beam along a path coincident (or parallel in certain special situations as in a presbyopic off-center ablation pattern center or a surgeon""s override) to a tilted eye""s designated tilt reference axis (e.g., the optical axis of the eye). This embodiment preferably features a first optical path directing device (preferably fixed) that directs the laser beam to a second optical path directing device preferably in the form of an adjustable scan mirror which delivers the beam and then to a third optical path device that includes a curved, ellipsoidal mirror. The scan mirror is positioned at the focal point of the ellipsoidal mirror (preferably a one quarter section of an ellipsoid) with the combination of the ellipsoidal mirror and scan mirror being oriented so as to cover the entire surgical ablation area common for eye surgery. The first directing device is helpful to enable the proper positioning of the laser beam onto the adjustable scan mirror located at one of the focal points of the ellipsoidal mirror. The ellipsoidal mirror will direct all incoming light rays toward a second focal point of the ellipsoidal mirror which coincides with the base point of the cornea""s radius. The ellipsoidal mirror is also dimensioned and arranged to cover the entire cone area of tilt (with the cone""s central axis coinciding with a non-tilt orientation of the eye which typically is the reference setting for the laser beam delivery system and the base of the cone preferably being along the iris plane). The ellipsoidal mirror associated with this laser delivery system thus negates the tilt of the eye in beam delivery so as to require only a determination as to where, on an X-Y plane, the laser beam axis should be directed. That is, under the present invention, a particular point on the curved mirror corresponds to a particular point on the cone base of tilt possibilities and is at angle directed at the cornea""s focal point. Accordingly, the eyetracking system merely needs to assign an X-Y plane point to the laser delivery system""s control to have the beam traveling off the curved mirror coincide with any eyetracker determined tilt orientation of the eye. Thus, for example, a large beam spot application having a predetermined pattern (based on, for instance, the use of a mask in line with the beam such as a mechanical iris device, a molded plastic material beam absorption mask insert, or an active pixel based mask as described below, etc.) can be applied with greater accuracy in a tilted eye situation so as to achieve greater conformity with the intended and resultant final ablation in the eye.
Another laser delivery system that can take advantage of the additional, accurate eye tilt information is an active mask system such as those described in co-pending U.S. Ser. Nos. 09/598,226 and 09/598,227 each filed on Jun. 21, 2000. These active mask systems are pixel based systems that can readily turn off or on desired pixels to achieve desired resultant ablation volume patterns in the eye. Thus, if a tilt determination is made, adjustments can be made in the pixel pattern presented to the laser beam to compensate for any energy variation that would arise if the beam was presented to an eye that is tilted. Thus, through coordination of the eyetracker system with the pixel pattern setting system described in the above application energy variation adjustments can be made in the pixels prior to the next laser beam pulse. A preferred embodiment features a pixel based mask like those described above which can provide different levels of transmissivity amongst the pixels to apply an energy pattern directed at negating any eye tilt.
The present invention is also directed at an initial patient positioning system that has an automated feature with a preferred embodiment having the eyetracker system utilize the reference markers in conjunction with a focusing control of an eye viewing device such as the eyetracker camera or some other camera or eye image viewing means including a surgical view video camera or the surgeons microscope. Through a series of sequential focusing steps with respect to, for example, the reference pattern of the present inventions reference marking means in coordination with adjustment in the patient bed movement system, the laser system can properly position the patient""s eye at a desired reference setting close to or at the proper laser beam start position. This is accomplished by first providing an input that will enable the bed to automatically move to a pre-established setting (e.g., a memorized setting from an earlier surgical procedure) that brings the patient""s head within a camera""s general view field. The surgeon also inputs an indication as to whether the right eye or left eye is being treated (an OD or OS command). Once the bed has moved to the general field location and the OD or OS information provided, the eyetracker system, in conjunction with the focusing means of the cornea and the bed adjustment means, carries out a fully automated sequence wherein, through a loop sequence of focus determinations and bed adjustments (and/or tilt pillow head holding means), the eye can be adjusted to at (or essentially at) the laser""s reference settings.
In a preferred embodiment of the invention the reference marking means utilize visible light beams that are strategically positioned for the purposes of reference information, and also to provide the lighting required by the eyetracker camera and microscope for proper functioning. Accordingly, the present invention provides the refractive surgeon with a very precise eyetracker that utilizes the reference marker lights as coaxial and/or auxiliary illumination lights for illuminating the operating field without the limitations and inconvenience seen in the previous infrared side illumination systems like infrared power changes, infrared contamination and infrared video noise generated by the activations and control of the microscope illumination and auxiliary lights when used by the surgeon during the execution of a refractive surgery.
Also, contrary to prior art systems that restrict illumination, the present invention is designed so as to encourage and even suggest with this system the use of excess illumination to enhance the detail view of the surgery for the surgeon and to greater the color contrast for the peak video detection of the different video eye targets comprised of limbo, iris, pupil and the reference lights.
The contrast and power balance is, for example, controlled and balanced under the present invention via an electronic automatic gain control (AGC) feed back in the camera to maintain the same working threshold regardless of the illumination conditions.
The present invention thus further provides a surgeon with an active eyetracker system able to detect changes not only for the X and Y plane as in the prior art, but also changes in the tilt of the eye and make the compensations via the ablation delivery system. Also, as discussed above, the present invention also features laser delivery systems that can take advantage of the tilt determination capability of the eyetracking system of the present invention.